As for a car, a mounted buttery serves as power supplies of mounted instruments. A vehicle mounts many instruments, and these instruments operate according to the situation while driving. Therefore, a power supply voltage of the mounted buttery considerably fluctuates compared with a power supply of a general electrical instrument. Furthermore, when the mounted buttery is cut off for some reasons while an engine is driven, it is known that a transient phenomenon called load dump is generated, and a high voltage is generated on a power supply line.
Therefore, conventionally, there is a technique proposed to protect the instruments mounted on the vehicle from an overvoltage such as a surge superimposed on the power supply line of the mounted buttery (refer to Patent Literature 1, for example).
The above conventional technique is configured such that when the surge is generated on the power supply line, a controlling means puts a blower motor circuit into an energized state while the surge is above a predetermined voltage. As a result, the surge flows in the blower motor circuit to be absorbed. The conventional technique reduces a burden of a surge absorbing circuit and the number of surge absorbing elements in this manner.
However, the above-described technique is configured such that when the surge is generated while a blower motor is stopped, the blower motor is energized and driven, and surge energy is absorbed by the blower motor. Therefore, when the surge is generated while the blower motor is energized and driven, the problem is that the surge energy cannot be absorbed more than the surge energy due to the energizing and driving.